Optical waveguide apparatus and method of producing the same

ABSTRACT

An optical waveguide apparatus comprises a substrate ( 1 ), a patterned waveguide as a core ( 4 ), and an upper cladding layer ( 10 ) formed on the substrate. The core is surrounded by a cladding comprising the substrate as a lower cladding layer and the upper cladding layer and smaller in refractive index than the core. The core and the cladding are integrally coupled to each other in a manner such that temperature-dependent expansion or contraction is performed substantially in accordance the characteristic of the cladding. The core and the cladding are made of materials selected so that the variation in optical path length according to the temperature-dependent expansion or contraction of the cladding is canceled by the variation in optical path length according to temperature-dependent variation in refractive index of the core.

BACKGROUND OF THE INVENTION

[0001] This invention relates to an optical waveguide apparatus for usein a wavelength division multiplex (WDM) optical communication system aswell as a method of producing the same. More specifically, thisinvention relates to a planar optical waveguide device for implementinga wavelength selecting function, such as an arrayed waveguide grating(AWG) used as an optical signal multiplexer or demultiplexer, as well asa method of producing the same.

[0002] In recent years, a wavelength division multiplex (WDM)transmission system becomes widely used in optical transmission. In theWDM transmission system, a number of signals different in wavelengthfrom one another are multiplexed and transmitted through a singleoptical fiber. As a greater number of signals are multiplexed, atransmission capacity is increased. Most recently, 100 or more signalsdifferent in wavelength are multiplexed. As a consequence, a separationor spacing between different wavelengths is narrowed. For example, in asystem of a 100 GHz grid, the spacing between two adjacent wavelengthsmust be equal to 0.8 nm. The WDM transmission system is initially usedin a long-distance network but is growing wider applications covering aperiphery of a terminal.

[0003] In the above-mentioned WDM transmission system, a device having awavelength selecting function of selecting a particular signal among anumber of signals different in wavelength is essential andindispensable. Such wavelength selecting function is provided by aplanar optical waveguide device as an integrated device.

[0004] As an example of the planar optical waveguide device having sucha wavelength selecting function, an arrayed waveguide grating (AWG) isdisclosed in Japanese Patent No. 2599786 (JP 2599786 C). The arrayedwaveguide grating is used as an optical multiplexer/demultiplexer.Referring to FIG. 1, a waveguide pattern of silica-based glass is formedon a substrate 1. The waveguide pattern includes at least one opticalinput waveguide 2, an input-side slab waveguide 3 as a first slabwaveguide, a plurality of patterned or arrayed waveguides (channelwaveguides) 4 different in length from one another, an output-side slabwaveguide 5 as a second slab waveguide, and at least one optical outputwaveguide 6 (in the illustrated example, a plurality of optical outputwaveguides 6 are shown) which are successively connected in this order.A combination of the arrayed waveguides 4 forms a diffraction grating 14so that the arrayed waveguide grating is provided. For simplicity ofillustration, only a small number of waveguides are shown in FIG. 1. Inan actual device, the arrayed waveguides are equal in number to about100. The number of the optical output waveguides corresponds to thenumber of output channels.

[0005] The optical input waveguide 2 is connected to an optical fiber(not shown) so as to introduce a wavelength-multiplexed light beam. Thelight beam introduced through the optical input waveguide 2 into theinput-side slab waveguide 3 is spread due to a diffracting effect of theinput-side slab waveguide 3 to be incident to the respective arrayedwaveguides 4 as split beams which propagate through the respectivearrayed waveguides 4. The split beams propagating through the respectivearrayed waveguides 4 reach the output-side slab waveguide 5. The splitbeams reaching the output-side slab waveguide 5 are condensed or focusedas a focused beam which propagates into the optical output waveguides 6to be outputted therefrom.

[0006] In the arrayed waveguide grating described above, the arrayedwaveguides 4 are different in length from one another. Therefore, afterthe split beams delivered from the input-side slab waveguide 3 propagatethrough the respective arrayed waveguides 4, the split beams are shiftedor differed in phase from one another. Depending upon the magnitude(quantity) of the phase shift or difference, the wavefront of thefocused beam is tilted. A focusing position is determined by the tiltingangle of the wavefront of the focused beam. Therefore, by forming theoptical output waveguides 6 at that position, output light beamsdifferent in wavelength from one another can be produced from theoptical output waveguides 6 corresponding to the different wavelengths,respectively.

[0007] In the arrayed waveguide grating, the diffraction grating 14 hasa wavelength resolution proportional to a difference (ΔL) in lengthbetween the arrayed waveguides 4 forming the diffraction grating 14.Therefore, by designing the diffraction grating 14 with a greater valueof ΔL, it is possible to carry out optical multiplexing anddemultiplexing for multiple light beams at a narrower wavelengthspacing.

[0008] However, in the above-mentioned arrayed waveguide grating, thepatterened waveguides 4 are different in length from one another. Thismeans that variations in length (optical path length) of the arrayedwaveguides 4 in response to the variation in device temperature aredifferent from one another. Therefore, in response to the variation indevice temperature, filtered wavelengths, i.e., wavelengthsdemultiplexed by the arrayed waveguides 4 are greatly changed.

[0009] In order to solve the above-mentioned problem, it is proposed tointroduce a temperature control mechanism into the opticalmultiplexer/demultiplexer. The temperature control mechanism comprises aPeltier device for cooling and a temperature control circuit and carriesout temperature control of the arrayed waveguide grating so that thetemperature variation itself is eliminated. However, introduction ofsuch a temperature control mechanism results in an increase in size ofthe apparatus, an increase in cost, and an increase in powerconsumption.

[0010] As another approach without using the Peltier device, proposal ismade of a method which will hereinafter be described in conjunction withFIG. 1. In order to cancel the temperature dependence of the arrayedwaveguides 4 of the arrayed waveguide grating, a trapezoidal groove isformed across the arrayed waveguides 4, as depicted by a broken line inFIG. 1. The arrayed waveguides 4 comprise silica-based glass coreshaving a positive temperature coefficient of refractive index. Atemperature compensating part 9 is formed by filling the trapezoidalgroove with silicone resin having a negative temperature coefficient ofrefractive index.

[0011] By canceling the variation in optical path length due to thetemperature-dependent variation in refractive index of each arrayedwaveguide 4, it is possible to remove the temperature dependence of thearrayed waveguide grating (see “Athermal silica-based arrayed-waveguidegrating (AWG) multiplexer”, ECOC '97 Technical Digest, pp. 33-36, 1997).In this approach, the temperature dependence of the transmissionwavelength of the arrayed waveguide grating is reduced to a small valueequal to 0.001 nm/° C. or less.

[0012] However, with the above-mentioned structure, optical mismatch iscaused between the arrayed waveguides and the temperature compensatingpart filled with silicone resin. Furthermore, it is difficult to form acladding layer on the temperature compensating part 9 of a trapezoidalshape formed in the arrayed waveguide grating. This brings aboutoccurrence of excessive loss in a region of the temperature compensatingpart 9 of a trapezoidal shape. As a consequence, the opticaltransmission loss characteristic of the arrayed waveguide grating as awhole device is degraded.

[0013] As a still another approach, EP 0849231 A1 discloses a method ofimproving the temperature characteristic by selecting a material of thewaveguide. This method aims to improve the temperature characteristic ofthe device resulting from the difference in temperature dependencebetween the waveguides different in material. By exactly matching theoptical path length temperature-dependent variation rate of twowaveguides, the temperature characteristic of the wavelength controlfunction is improved.

[0014] However, the above-mentioned method is not applicable to a devicesuch that the waveguides are made of a same material and the wavelengthcontrol characteristic is achieved by the difference in physical lengthbetween the waveguides. Furthermore, even if the temperaturecharacteristics of the individual waveguides are rendered identical bythe use of the waveguides of the same material, a desired wavelengthcontrol characteristic can not be achieved unless the temperaturecharacteristics of the individual waveguides are sufficiently low.

[0015]FIG. 2 shows a production process of a ridged optical waveguidewidely used. In a first step, a core layer 4 a is formed on thesubstrate 1. In a second step, the core layer 4 a is patterned by alithography or the like to form a plurality of cores 4. In a third step,an upper cladding layer 10 is formed to cover the cores 4. Thus, thecores 4 are surrounded by the upper cladding layer 10 and the substrate1. Therefore, the substrate 1 may be called a lower cladding layer. Acombination of the upper and the lower cladding layers may becollectively referred to as a cladding surrounding the cores 4.

[0016] Herein, glass thin films as the core layer 4 a and the uppercladding layer 10 are formed by flame hydrolysis deposition. In casewhere the thin film is formed by the flame hydrolysis deposition, heattreatment is required after the thin film is formed. This is because aresultant deposit (called a soot) obtained by the flame hydrolysisdeposition is low in density and must be increased in density in orderto achieve excellent optical characteristics and low propagation loss.Therefore, in case where a planar waveguide device of a multilayerstructure is formed by the above-mentioned thin film forming technique(flame hydrolysis deposition), a structure formed in a later step of theproduction process must have a glass transition point lower than that ofa structure formed in an earlier step. Specifically, a glass of theupper cladding layer must have a glass transition point lower than thoseof a core glass and the substrate. As a consequence, the upper claddinglayer 10 and the substrate 1 as the lower cladding layer are made ofdifferent glass materials. In this case, the upper and the lowercladding layers are different in coefficient of thermal expansion. Thisresults in undesired stress applied to the core 4 as the opticalwaveguide.

[0017] As another production process, it is possible to form the uppercladding layer 10 by the use of chemical vapor deposition (CVD). In thiscase, the upper cladding layer 10 can be formed by the material same asthat of the substrate 1 as the lower cladding layer. However, cracks orvoids are locally formed between the upper cladding layer 10 and thecores 4. Thus, the upper cladding layer 10 and the cores 4 are notalways kept in mechanical tight contact with each other.

[0018] After the thin film is formed, the heat treatment is required toremove the cracks or the voids formed in the upper cladding layer 10. Incase where the heat treatment is carried out under such conditions thatthe cracks or the voids can be removed, not only the lower claddinglayer formed by the same material is deformed but also the cores 4typically lower in softening point are deformed. Therefore, it isdifficult to achieve mechanical bond between the cladding and the coreby the heat treatment.

[0019] As described above, the conventional arrayed waveguide gratingshave various disadvantages. Furthermore, the existing methods ofproducing an optical waveguide apparatus such as the arrayed waveguidegrating have several disadvantages also. These disadvantages are notrestricted to the arrayed waveguide grating but apply to other typicaloptical waveguide apparatuses.

SUMMARY OF THE INVENTION

[0020] It is therefore an object of this invention to provide an opticalwaveguide apparatus which is practically free from temperature-dependentvariation of an optical path length, low in optical transmission loss,small in size, and low in cost.

[0021] It is another object of this invention to provide a method ofproducing the optical waveguide apparatus mentioned above.

[0022] According to an aspect of this invention, there is provided anoptical waveguide apparatus comprising a substrate, a core formed in arecessed portion of the substrate, and an upper cladding layer formed onthe substrate, the core being surrounded by a cladding comprising thesubstrate as a lower cladding layer and the upper cladding layer andsmaller in refractive index than the core;

[0023] the core and the cladding being integrally coupled to each otherin a manner such that temperature-dependent expansion or contraction isperformed substantially in accordance with the characteristic of thecladding;

[0024] the core and the cladding being made of materials selected sothat the variation in optical path length of the core according to thetemperature-dependent expansion or contraction of the cladding iscanceled by the variation in optical path length according totemperature-dependent variation in refractive index of the core.

[0025] Preferably, each of the core and the cladding is made ofsilica-based glass. The core is made of a material having a negativetemperature coefficient of refractive index.

[0026] Preferably, each of the substrate and the upper cladding layer ismade of Ti-doped SiO₂ or F-doped SiO₂.

[0027] Preferably, the cladding is made of Ti-doped SiO₂ while the coreto be combined therewith is made of a material selected from thosehaving a smaller temperature coefficient of refractive index as comparedwith the case where the cladding is made of SiO₂.

[0028] Preferably, the cladding is made of F-doped SiO₂ while the coreto be combined therewith is made of a material selected from thosehaving a greater temperature coefficient of refractive index as comparedwith the case where the cladding is made of SiO₂.

[0029] Preferably, the core is made of a material containing B₂O₃.

[0030] Preferably, the core is made of a material selected from aSiO₂—GeO₂—B₂O₃ glass, a SiO₂—TiO₂—B₂O₃ glass, and a SiO₂—GeO₂—B₂O₃—P₂O₅glass.

[0031] Preferably, the contents of GeO₂ and B₂O₃ have a ratio of 2:1 to3:1.

[0032] Preferably, the cladding is made of a material selected fromthose having a refractive index lower than that of the material of thecore;

[0033] the cladding material and the core material being combined sothat the temperature coefficient of refractive index of the corematerial is different in sign from and equal in magnitude (absolutevalue) to the coefficient of thermal expansion of the cladding material.

[0034] According to another aspect of this invention, there is provideda method of producing an optical waveguide apparatus, comprising thesteps of:

[0035] forming a recessed portion having a predetermined pattern on asurface of a substrate;

[0036] forming a core in the recessed portion of the substrate by theuse of a material having a refractive index higher than that of thesubstrate, a temperature coefficient of refractive index which has avalue reverse in plus/minus sign to a coefficient of thermal expansionof the substrate, and a glass transition point lower than that of thesubstrate; and

[0037] heat-treating the substrate with the core formed in the recessedportion at a temperature higher than the glass transition point of thematerial of the core formed in the recessed portion of the substrate andlower than the glass transition point of the substrate.

[0038] Preferably, the predetermined pattern corresponds to an opticalwaveguide pattern forming an arrayed waveguide grating.

[0039] Preferably, the heat treating step is carried out at atemperature higher than the glass transition point of a core glass andlower than the glass transition point of SiO₂ as a cladding glass.

[0040] Preferably, the step of forming the core is followed by the stepof forming an upper cladding layer to cover an upper surface of thesubstrate and an upper surface of the core formed in the recessedportion of the substrate;

[0041] the step of forming the upper cladding layer being followed bythe step of heat-treating the substrate.

[0042] Preferably, the substrate and a glass plate to become the uppercladding layer are put into optical contact at normal temperature andthen heat treated at a temperature higher than the glass transitionpoint of the core material and lower than the glass transition point ofthe cladding material.

[0043] According to the above-mentioned aspects of this invention, it ispossible to provide the optical waveguide apparatus capable ofprincipally eliminating the temperature-dependent variation in opticalpath length. Specifically, the temperature-dependent variation of theoptical path length of the optical waveguide apparatus is theoreticallygiven by the following equation (1):

d(nL)/dT=nL{1/n(dn/dT)+1/L(dL/dT)}  (1)

[0044] where n represents a refractive index of the optical waveguide,L, a physical length of the optical waveguide, T, a temperature.Considering that most of energy of a propagating light beam concentratesto the interior of the core, n is substantially represented by therefractive index of a core material. From the above-mentioned logic, thevariation rate of the physical length L corresponds to the coefficientof thermal expansion of the core. If the core is mechanicallyconstrained by the substrate as the lower cladding layer and the uppercladding layer, the coefficient of thermal expansion of the core issubstantially represented by the coefficient of thermal expansion of acladding material because the upper and the lower cladding layers have avolume much greater than that of the core.

[0045] From the equation (1), it is understood that thetemperature-dependent variation in optical path length can be eliminatedwhen the term (1/n(dn/dT)) corresponding to the temperature coefficientof refractive index of the core material is reverse in plus/minus signand equal in value (absolute value) to the term (1/L(dL/dT))corresponding to the coefficient of thermal expansion of the claddingmaterial. Since the coefficient of thermal expansion of SiO₂ widely usedas the cladding material, the temperature coefficient of refractiveindex of the core material must have a negative value.

[0046] In case where the above-mentioned condition is implemented, forexample, by a planar optical waveguide, it is desired that a core ismechanically tightly constrained by the cladding comprising the upperand the lower cladding layers and surrounding the core. Therefore, inthe above-mentioned aspects of this invention, it is preferable that theglass substrate as the lower cladding layer is provided with therecessed portion in which the core is buried and that the upper claddinglayer is formed by the material same as that of the lower claddinglayer. In order to perform the heat treatment to achieve the mechanicaltight contact between the core and the cladding, it is essential thatthe glass transition point of the core material is sufficiently lowerthan that of the cladding. In the device of the above-mentionedstructure, the shape of the core is retained by the cladding having ahigher glass transition point. Therefore, after the core is formed, theshape of the core is not deformed by the heat treatment at a hightemperature. Furthermore, during the heat treatment after the uppercladding layer is formed, the core is not deformed. Thus, it is possibleto perform the heat treatment at a relatively high temperature toachieve the mechanical tight contact between the core and the cladding.

[0047] According to the above-mentioned aspects, it is possible topractically eliminate the temperature-dependent variation in opticalpath length without additionally using temperature stabilizing means,such as a Peltier device, or without inserting a material different intemperature coefficient of refractive index in the middle of an opticalpath. Thus, according to the above-mentioned aspects, it is possible toeasily obtain an optical waveguide apparatus which is practically freefrom temperature-dependent variation in optical path length, small inoptical transmission loss, small in size, and low in cost.

BRIEF DESCRIPTION OF THE DRAWING

[0048]FIG. 1 is a view showing the structure of a conventional arrayedwaveguide grating;

[0049]FIG. 2 is a view for describing a production process of a ridgedoptical waveguide widely used;

[0050]FIG. 3A is a view showing the structure of an arrayed waveguidegrating as an optical waveguide apparatus according to an embodiment ofthis invention;

[0051]FIG. 3B is a sectional view taken along a line B-B in FIG. 3A; and

[0052]FIG. 4 is a view for describing a production process of thearrayed waveguide grating as a production process of the opticalwaveguide apparatus according to an embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0053] Now, description will be made of an arrayed waveguide grating anda method of producing the same according to an embodiment of thisinvention with reference to the drawing.

[0054] Referring to FIG. 3A, a substrate 1 is provided with at least oneoptical input waveguide 2, an input-side slab waveguide 3 as a firstslab waveguide, a plurality of arrayed waveguides (channel waveguides) 4different in length from one another, an output-side slab waveguide 5 asa second slab waveguide, and at least one (typically, a plurality of)optical output waveguide 6 which are successively connected in thisorder. In the illustrated example, a plurality of the optical outputwaveguides 6 are shown.

[0055] A combination of the arrayed waveguides 4 forms a diffractiongrating 14. For simplicity of illustration, only a small number of thewaveguides are shown in FIG. 3A. In an actual device, the number of thearrayed waveguides 4 is equal to, for example, 93 and the number of theoptical output waveguides 6 is equal to, for example, 8 corresponding tothe number of output channels. Generally, it is sufficient that thenumber of the optical input waveguide 2 is equal to one. If a pluralityof the optical input waveguides 2, equal in number to the optical outputwaveguides 6 are provided, a waveguide structure is symmetrical so thatit is used from either side.

[0056] Referring to FIG. 3B, each of the arrayed waveguides 4 comprisesa core material filled in a groove formed in the substrate 1 as a lowercladding layer. An upper cladding layer 10 same in material to thesubstrate 1 is integrally formed with the core material. Thus, astructure in which the core is surrounded by a cladding comprising thesubstrate 1 as the lower cladding layer and the upper cladding layer 10is obtained. In the illustrated example, the upper cladding layer 10 hasa thickness of 1 mm and the arrayed waveguide 4 has a dimension of 7μm×7 μm in section. A space between two adjacent ones of the arrayedwaveguides has a width equal to 4 μm at a narrowest position. Each ofthe substrate 1 as the lower cladding layer and the upper cladding layer10 is formed by SiO₂. The arrayed waveguide 4 as the core is formed bySiO₂—GeO₂—B₂O₃ glass (GeO₂: 13 mol %, B₂O₃: 6 mol %, SiO₂ : 81 mol %)).

[0057] Each of the optical input waveguide 2, the optical outputwaveguides 6, the input-side slab waveguide 3, and the output-side slabwaveguide 5 is formed by a glass material same as that of the arrayedwaveguide 4. The glass material of the above-mentioned composition has anegative temperature coefficient of refractive index such that thevariation in optical path length of the core due to thetemperature-dependent variation of SiO₂ geometric size is substantiallycanceled. Specifically, the temperature coefficient of refractive indexof a core glass material is different in plus/minus sign from thecoefficient of thermal expansion of SiO₂. In the equation (1), the term(1/n(dn/dT)) corresponding to the temperature coefficient of refractiveindex of the core glass material has a value (absolute value)substantially equal to a value (absolute value) of the term (1/L(dL/dT))corresponding to the coefficient of thermal expansion of SiO₂. It isnoted here that the glass transition point of SiO₂, i.e., the glasstransition point of the cladding material is equal to about 1190° C. Onthe other hand, the core glass material has a glass transition point ofabout 1050° C. which is lower than the glass transition point of SiO₂.

[0058] Selection of the core material and the cladding material will beconsidered. By way of example, description will be made of the casewhere each of the core material and the cladding material is glass. Atfirst, as will later be described, the core forming the arrayedwaveguide 4 must be uniformly and entirely kept in tight contact withand constrained by the substrate 1 forming the lower layer and the uppercladding layer 10. For this purpose, the heat treatment is carried outafter the core is formed in the recessed portion to form the arrayedwaveguide 4. The heat treatment must be performed at a temperature notlower than the glass transition point of the core glass material so asto achieve the above-mentioned constrained state. Therefore, the glasstransition point of the cladding material must be equal to or higherthan the glass transition point of the core glass material.

[0059] Next, a combination of the cladding glass material and the coreglass material is selected so that, in addition to the relationshipbetween the glass transition points, the core glass material has apredetermined refractive index and a predetermined temperaturecoefficient of refractive index while the cladding glass materialsatisfies a predetermined coefficient of thermal expansion. At first,depending upon the shape of the optical waveguide, the refractive indexrequired to the core material is determined. Generally, as the opticalwaveguide is smaller in size, the core material is required to have ahigher refractive index. Next, in order that the light beam is confinedin the optical waveguide and propagates therethrough, the claddingmaterial is selected from those materials lower in refractive index thanthe core material. At this time, the combination is selected so that, inthe equation (1), the term (1/n(dn/dT)) corresponding to the temperaturecoefficient of refractive index of the core glass material is reverse inplus/minus sign and equal in value (absolute value) to the term(1/L(dL/dT)) corresponding to the coefficient of thermal expansion ofthe cladding glass material.

[0060] Hereinafter, specific examples of the cladding glass material andthe core glass material will be mentioned. As the material of each ofthe substrate 1 and the upper cladding layer 10, use may be made ofTi-doped SiO₂ and F-doped SiO₂ in addition to SiO₂. By doping Ti intoSiO₂, the refractive index of the glass is increased and the coefficientof thermal expansion is decreased. Therefore, in case where Ti-dopedSiO₂ is used as the cladding material, the core material to be combinedtherewith is selected from those materials having a small temperaturecoefficient of refractive index as compared with the case where SiO₂ isused as the cladding material. Furthermore, the core material must havea relatively high refractive index.

[0061] On the other hand, in case where F is doped into SiO₂, therefractive index of the glass is decreased and the coefficient ofthermal expansion is increased. In this case, the core material to becombined is selected from those materials having a large temperaturecoefficient of refractive index as compared with the case where SiO₂ isused as the cladding material, contrary to the above-mentioned case. Thetolerance or allowance of the refractive index is widened. As describedabove, the cladding material satisfying the desired characteristics mustbe appropriately selected so that the right side of the above equation(1) is equal to 0, considering the refractive index of the corematerial, the temperature coefficient of refractive index, and thecoefficient of thermal expansion of the cladding material.

[0062] As an example of the core material of the arrayed waveguide 4 asthe core, use may be made of SiO₂—GeO₂—B₂O₃ glass, SiO₂—TiO₂—B₂O₃ glass,and SiO₂—GeO₂—B₂O₃—P₂O₅ glass. Among components of these glasses, GeO₂and TiO₂ increase the refractive index of the glass while B₂O₃ and P₂O₅decrease the temperature coefficient of refractive index of the glass.

[0063] Generally, as the content of B₂O₃ is increased, weatherresistance of the glass is degraded. However, if B₂O₃ and P₂O₅ are addedtogether, the effect of improving the stability is obtained. By way ofexample, consideration will be made of the SiO₂—GeO₂—B₂O₃ glass. Byaddition of GeO₂, the refractive index of the glass is increased but,simultaneously, the temperature coefficient of refractive index isincreased. Therefore, in order to obtain a desired temperaturecoefficient of refractive index, the content of B₂O₃ must be increasedas the content of GeO₂ is increased. In view of the above, the ratio ofGeO₂ and B₂O₃ is preferably between 2:1 and 3:1.

[0064] Referring to FIG. 4, description will be made of a method ofproducing the arrayed waveguide grating having the above-mentionedstructure. On the synthetic silica-based glass (SiO₂) substrate 1 havinga thickness of 1 mm and a diameter of 3 inches, a recessed groove 40corresponding to the optical waveguide pattern forming the arrayedwaveguide grating is formed. The recessed groove 40 corresponding to thearrayed waveguide 4 has a width of 7 μm and a depth of 7 μm. The portionhaving a smallest space (width of a portion between two adjacent ones ofthe arrayed waveguides) is formed in the vicinity of a joint portionbetween the arrayed waveguides 4 and the slab waveguide 5 and is equalto 4 μm.

[0065] The recessed groove 40 is formed by the use of photolithographywidely used in LSI production or the like. Etching is carried out byreactive ion etching using Freon 23 (CHF₃) (hereinafter simply referredto as Freon). In order to form the recessed groove having an exactsquare section, an induction coupling plasma (hereinafter abbreviated toICP) apparatus (manufactured by Samco) is used as a reactive ion etchingapparatus. A Freon gas of about 5 mTorr is introduced as a reactive gas.High-frequency power is applied under the condition of a frequency of13.56 MHz and a power of 103W. A Cr thin film formed by sputtering andhaving a thickness of 100 nm is used as a mask.

[0066] Next, the Cr mask left on the substrate 1 is removed by the useof cerium ammonium nitrate ((NH₄)₂Ce(NO₃)₆). Thereafter, a glass thinfilm 4 a to serve as the core is formed on the substrate 1. As the core,an amorphous hydrogenated silicon dioxide (a-SiO₂:H) film with germanium(Ge) and boron (B) added thereto is deposited by plasma enhancedchemical vapor deposition (hereinafter abbreviated to PECVD). Materialgases are tetraethoxy silane (Si(OC₂H₅)₄: hereinafter abbreviated toTEOS), tetramethoxy germane (Ge(OCH₃)₄: hereinafter abbreviated toTMOG), triethoxy borane (B(OC₂H₅)₃), and oxygen (O₂).

[0067] In the above-mentioned material, triethoxy boron may be replacedby trimethoxy boron (B(OCH₃)₃). By changing the material gas, the dopingamount contained in the deposited film is varied. In the above-mentioneddeposition, the desired composition is obtained by optimizing thedepositing condition such as a gas pressure. In this case, the gaspressure in a vacuum chamber during deposition is equal to 5.0 Pa. Thehigh-frequency power supplied to an ICP reactor and a substrateelectrode is equal to 900W and 300W, respectively. A VDC at thesubstrate electrode arranged on the surface of the substrate is equal to−400V. The substrate temperature is 250° C. By controlling the flow rateof the material gas, the thin film 4 a comprising 12.5 mol % germaniumoxide, 6.2 mol % boron oxide (B₂O₃), and 81.3 mol % silicon oxide isobtained. The deposition time is 120 minutes and the depositionthickness is 7 μm.

[0068] It is widely known that cracks or voids are locally observed inthe glass formed in the recessed groove 40 and the glass is not entirelykept in tight contact with the substrate 1 as the lower cladding layer.In order to avoid the above-mentioned problem as far as possible, thethin film is formed by the use of plasma enhanced chemical vapordeposition in this embodiment. In order to achieve the state where thecore is uniformly and entirely constrained by the lower cladding layerformed by the substrate 1, heat treatment is carried out. The heattreating condition is 1100° C. and 30 minutes in this embodiment. Theabove-mentioned temperature is higher than the glass transition point ofthe core glass and lower than the grass transition point of SiO₂ as thecladding glass. The heat treatment is also effective in suppressing thefluctuation of the refractive index and in removing hydrogen duringdeposition of the thin film.

[0069] In this embodiment, an unnecessary part of the thin film attachedto an area except the groove is removed by polishing to leave the coreso that the arrayed waveguide 4 is formed. To the substrate 1 with anoptically flat surface achieved by polishing, a silica (SiO₂) glassplate having a thickness of 1 mm is bonded as the upper cladding layer10. The bonding condition is as follows. After the substrate 1 and theglass plate as the upper cladding layer 10 are brought into opticalcontact, heat treatment is carried out at 1100° C. for 30 minutes. Theabove-mentioned temperature is higher than the glass transition point ofthe core glass material and lower than the glass transition point ofSiO₂ as the cladding glass. As a consequence, the two glass plates asthe substrate 1 and the upper cladding layer 10 are fully bonded andintegrated so that no boundary is observed therebetween. The upper partof the core is brought into tight contact with the upper cladding layer10. The core is integrally coupled with the cladding comprising theupper and the lower cladding layers surrounding the core.

[0070] As described above, the production process including a series ofsteps and mainly utilizing the thin-film/lithography technique iscompleted. On the single substrate, four arrayed waveguide gratings aresimultaneously formed. In order to obtain a device which can be used asa component, each arrayed waveguide grating circuit is separated fromthe substrate. Then, each of the input waveguide 2 and the outputwaveguides 6 is connected to a fiber. A signal is inputted and outputtedby the use of the fibers. When the fibers are mounted, the device iscompleted.

[0071] The arrayed waveguide grating apparatus produced as mentionedabove is operable in a wavelength band of 1.55 μm. The channel spacingis 1.6 nm (200 GHz grid) and the number of channels is equal to 1×8.Next, in order to examine the temperature characteristic, a wavelengthvariable laser is connected to one input port and a signal beam emittedfrom a particular output port is measured. After the whole device is putinto an environment tester capable of exactly controlling thetemperature and the humidity, the temperature is elevated stepwise. Whensufficient thermal equilibrium is achieved, the wavelength of the outputsignal beam is measured. In case where the temperature is elevated from0° C. to 85° C., the total variation of the center wavelength of thearrayed waveguide grating apparatus in this embodiment is equal to 0.05nm.

[0072] According to the reported data, the variation of the centerwavelength in the arrayed waveguide grating apparatus of the existingstructure is equal to 0.012 nm/° C. The variation over the temperatureelevation from 0° C. to 85° C. is assumed to be equal to 1.02 nm. Ascompared with a channel spacing of 1.6 nm, the temperature-dependentvariation of the center wavelength in the existing device brings about alarge loss so that practical use is difficult.

[0073] In recent years, following the increase in communicationcapacity, the channel spacing tends to be narrower. At present, thechannel spacing of 0.8 nm (100 GHz grid) is becoming a major trend. Inthis case also, selection of the material and production of the arrayedwaveguide grating can be carried out in the manner similar to thearrayed waveguide grating having a channel spacing of 1.6 nm. Thenarrower channel spacing results in a more and more strict requirementupon the temperature-dependent variation of the center wavelength. Incase of the spacing of 0.8 nm, it is required for the device that thevariation of the center wavelength over the range from 0 to 85° C. isnot greater than about 50 pm. In this respect also, the opticalwaveguide apparatus of this invention suppressed in temperaturedependence is useful.

[0074] As described above, according to this invention, the core and thecladding are integrally coupled in a manner such thattemperature-dependent expansion or contraction is performedsubstantially in accordance with the characteristic of the cladding. Thematerials of the core and the cladding are selected so that thevariation in optical path length according to the temperature-dependentexpansion or contraction of the cladding is canceled by the variation inoptical path length according to the temperature-dependent variation inrefractive index of the core. Thus, it is possible to obtain the opticalwaveguide apparatus which is practically free from temperature-dependentvariation in optical path length, small in optical transmission loss,small in size, and low in cost as well as the method of producing thesame.

What is claimed is:
 1. An optical waveguide apparatus comprising asubstrate, a core formed in a recessed portion of said substrate, and anupper cladding layer formed on said substrate, said core beingsurrounded by a cladding comprising said substrate as a lower claddinglayer and said upper cladding layer and smaller in refractive index thansaid core; said core and said cladding being integrally coupled to eachother in a manner such that temperature-dependent expansion orcontraction is performed substantially in accordance with thecharacteristic of said cladding; said core and said cladding being madeof materials selected so that the variation in optical path length ofsaid core according to the temperature-dependent expansion orcontraction of said cladding is canceled by the variation in opticalpath length according to temperature-dependent variation in refractiveindex of said core.
 2. An optical waveguide apparatus as claimed inclaim 1, wherein each of said core and said cladding is made ofsilica-based glass, said core being made of a material having a negativetemperature coefficient of refractive index.
 3. An optical waveguideapparatus as claimed in claim 1, wherein each of said substrate and saidupper cladding layer is made of Ti-doped SiO₂ or F-doped SiO₂.
 4. Anoptical waveguide apparatus as claimed in claim 3, wherein said claddingis made of Ti-doped SiO₂ while said core to be combined therewith ismade of a material selected from those having a smaller temperaturecoefficient of refractive index as compared with the case where saidcladding is made of SiO₂.
 5. An optical waveguide apparatus as claimedin claim 3, wherein said cladding is made of F-doped SiO₂ while saidcore to be combined therewith is made of a material selected from thosehaving a greater temperature coefficient of refractive index as comparedwith the case where said cladding is made of SiO₂.
 6. An opticalwaveguide apparatus as claimed in claim 2, wherein said core is made ofa material containing B₂O₃
 7. An optical waveguide apparatus as claimedin claim 6, wherein said core is made of a material selected from aSiO₂—GeO₂—B₂O₃ glass, a SiO₂—TiO₂B₂O₃ glass, and a SiO₂—GeO₂—B₂O₃—P₂O₅glass.
 8. An optical waveguide apparatus as claimed in claim 7, whereinthe contents of GeO₂ and said B₉O₃ have a ratio of 2:1 to 3:1.
 9. Anoptical waveguide apparatus as claimed in claim 1, wherein said claddingis made of a material selected from those having a refractive indexlower than that of the material of said core; said cladding material andsaid core material being combined so that the temperature coefficient ofrefractive index of said core material is different in sign from andequal in magnitude (absolute value) to the coefficient of thermalexpansion of said cladding material.
 10. A method of producing anoptical waveguide apparatus, comprising the steps of: forming a recessedportion having a predetermined pattern on a surface of a substrate;forming a core in the recessed portion of said substrate by the use of amaterial having a refractive index higher than that of said substrate, atemperature coefficient of refractive index which has a value reverse inplus/minus sign to a coefficient of thermal expansion of said substrate,and a glass transition point lower than that of said substrate; andheat-treating said substrate with said core formed in said recessedportion at a temperature higher than the glass transition point of thematerial of said core formed in said recessed portion of said substrateand lower than the glass transition point of said substrate.
 11. Amethod as claimed in claim 10, wherein said predetermined patterncorresponds to an optical waveguide pattern forming an arrayed waveguidegrating.
 12. A method as claimed in claim 10, wherein said heat treatingstep is carried out at a temperature higher than the glass transitionpoint of a core glass and lower than the glass transition point of SiO₂as a cladding glass.
 13. A method as claimed in claim 10, wherein: thestep of forming said core is followed by the step of forming an uppercladding layer to cover an upper surface of said substrate and an uppersurface of said core formed in said recessed portion of said substrate;the step of forming said upper cladding layer being followed by the stepof heat-treating said substrate.
 14. A method as claimed in claim 13,wherein said substrate and a glass plate to become said upper claddinglayer are put into optical contact at normal temperature and then heattreated at a temperature higher than the glass transition point of thecore material and lower than the glass transition point of the claddingmaterial.